#57 Undo in channel topic field

[filippl]

Changes in the topic field should be undoable.

Current behaviour:
Change the topic, don't press Enter, the topic shows as
changed only for the current user (this is OK). Hit
Cmd-Z, the topic is not reverted. The only way to roll
back to the revious state is to rejoin the channel.

Expected behaviour:
If the user changes the topic, but doesn hit Enter,
make it possible to revert to the previous topic (Cmd-Z)